Numerically controlled oscillators have been the subject of discussion in literature for many years. For example, an article entitled "A Digital Frequency Synthesizer" by Tierney, Rader and Gold was published in IEEE Trans. Audio Electroacustics, vol AU-19, pp 48-56 Mar., 1971. This article is illustrative of early interest in NCO circuits. However, prior to the availability of reasonably low cost integrated circuits, numerically controlled oscillators were too expensive for many applications. Integrated circuitry has reduced the cost of NCO circuits and has improved their performance. For example, there are commercially available NCO circuits which operate at clock rates up to 60 mHz. The output frequency of an NCO is a direct function of clock frequency; and the practical upper limit of output frequency of an NCO is about 45% of the clock frequency. Accordingly, there is substantial commercial incentive to increase clock frequency so as to reach new markets.
A numerically controlled oscillator comprises a "phase accumulator" which is incremented at the frequency of the clock signal to provide multibit digital word output signals in sequence.
In a typical application, the digital output words are employed to address a waveform lookup table i.e., a sine or cosine table in memory. The output signals of a lookup table (LUT) are series of digital words which correspond in value to the amplitudes of samples of the generated output signal. In such arrangements the speed of operation of the LUT limits the overall speed of operation of the system.